WebService::Xero 0.11 and earlier for Perl uses the rand() function as the default source of entropy, which is not cryptographically secure, for cryptographic functions. Specifically WebService::Xero uses the Data::Random library which specifically states that it is "Useful mostly for test programs". Data::Random uses the rand() function.
https://security.metacpan.org/docs/guides/random-data-for-security.html
https://perldoc.perl.org/functions/rand
https://metacpan.org/release/LOCALSHOP/WebService-Xero-0.11/source/lib/WebService/Xero/Agent.pm#L178
https://metacpan.org/release/LOCALSHOP/WebService-Xero-0.11/source/lib/WebService/Xero/Agent.pm#L17
https://metacpan.org/release/BAREFOOT/Data-Random-0.13/source/lib/Data/Random.pm#L537